I started a brew sunday evening, it is supposed to be a dark brown english ale.

When I pitched the yeast it was a perfect 21C (70F), I went to bed, checked it the next morning and the temp had risen to 25C (77F) (which is the upper limit for the yeast I am using) so I opened up the blanket a bit and went to work. when I came home the temp had risen to 30C+ (86F+), it had maxed out my stick on thermometer.

You will definitely wind up with some off flavors at those temperatures and being pretty extreme they may not dissipate or age out.

The actual active fermentation process has the ability to generate enough heat to raise temperatures a good 5-10 degrees as you are now finding out.

While a little tardy it would benefit the beer if you could get the temps down. An easy way to do this is to get a large enough tub, fill with cold water, place the vessel in it and add enough frozen water bottles to drop the temp into the 60's. It's called a swamp cooler and works very well!

As for the beer, definitely leave it on the yeast for a minimum of 3-4 weeks and give it a chance to condition and clean up. Some of the major fruity esters may dissipate a bit. the beer will not be a total loss but don't expect it to taste like it should. Drink it and learn about fermentation temperature control so your prepared for the next batch

21C is far from perfect for nearly any brew to pitch at, esp something english . like duboman pointed out, fermentation will raise the internal temp so you always want to be a few degrees below what you want to ferment at. I would not let an english strain get up to 21C even at height of ferment tho, not enjoyable results

I've gotten to this point where I feel like pitch temp is almost more important than fermentation temp. So I always pitch low, like 62 or so.

In reality what I mean is that if you pitch low, you get through most of growth (read: flavor-development stage) at a favorable temp, and by the time you're heavily fermenting the yeast is up to something like 68.

I've gotten to this point where I feel like pitch temp is almost more important than fermentation temp. So I always pitch low, like 62 or so.

In reality what I mean is that if you pitch low, you get through most of growth (read: flavor-development stage) at a favorable temp, and by the time you're heavily fermenting the yeast is up to something like 68.

This is mostly true, but a water bath further helps buffer the temp changes. Monitoring them closely and adding ice bottles if you can will help. Pick a temp you don't want to exceed and watch the rate of temp rise. As it even nears that temp, add ice bottles to stabilize it. There is going to be one really hot period in fermentation, then it will fall back down some, so don't let it fall too far or the yeast will fall out of suspension.

Pitch low, buffer temp changes, add ice bottles or cooling when it rises, then let it stabilize after the "hot phase" is over without much cooling.